Airlines and travel agents usually sell trip medical insurance. An example is American Express, but I don't know if they will cover a foreign visitor to the USA. Go to www.americanexpress.com, click on Travel, the Global Travel Shield for more info.
Any guest you have staying should purchase there travel insurance in there home country. Most insurance has restriction on the nationality of the user.
I travel to the states and have found that the UK travel insurance is much better value for money than what is available in the USA. $70 for six months covers and up to $10 million dollars worth of treatment with no surcharge